What is the UK government going to do in the face of mounting evidence that the Saudi Arabian-led air attacks on Yemen appear to be in breach of international law?

In the year since the bombing began in March 2015, the UK has sold £3.3bn in arms to Saudi Arabia. That is a huge boost for UK exports – the deal is for fighter planes and components, as well as bombs and missiles – and a guarantee of jobs at a time of economic uncertainty. Against that backdrop, the chances of the UK suspending arms sales are extremely slim.

And yet there have been few international issues since the 2003 invasion of Iraq that have created such a sense of unease in the UK as the scale of civilian casualties in Yemen.

What makes the issue even more controversial is that UK military advisers are based at the Saudi command and control headquarters where the air campaign against Houthi forces in Yemen is being conducted. The Ministry of Defence has been coy about precisely what their role is, details having to be teased out.

The unease manifested itself early on in the campaign when calls were put into media organisations by British expatriates based in Saudi Arabia and members of the public in the UK who had picked up snippets from British service personnel in pubs, clubs or school playgrounds about the UK military working alongside the Saudi air force.

Human rights groups and campaigners against the arms trade echoed the concerns – followed, increasingly, by MPs. Questions to government ministers have been steadily rising, either in the Commons chamber or through select committee hearings or parliamentary written questions.

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On Thursday alone, two Commons select committees – international development and business – called in a joint report for the suspension of arms sales to Saudi Arabia until a credible and independent investigation had been conducted into the number of civilian deaths from the air campaign. But the foreign affairs committee, chaired by the Tory MP Crispin Blunt, remains reluctant. Rather than calling for a suspension, it said that sales should only stop if UK courts ruled them unlawful.

The government’s official line, reflected in a parliamentary answer on Thursday, is there will be no suspension of sales.

A Yemeni child with a car destroyed by a Saudi airstrike. Photograph: Anadolu Agency/Getty Images

In private, though, there is an unusually high degree of self-doubt from at least some officials over whether Saudi Arabia has breached international law and whether the UK could be implicated. Some of this seeped out on the final day of parliament before the summer recess, when the government retracted earlier assured statements that the Saudis had not been in breach.

So what precisely does the small British contingent attached to the Saudi command and control centre do? An MoD spokesperson said the personnel were not directing operations or choosing targets, only providing advice, including on compliance with international law.

But a Whitehall source with knowledge of the situation was more forthcoming. “The Saudis explain only in broad terms what they intend to do and the British will respond along the lines of ‘Well, have you thought of this? Have you done that?’ And that is about it.”

The source added: “I don’t think the Saudis go out to kill civilians, but it would be fair to say the Saudis have different thresholds when it comes to tolerance of things like that.”

Some of this chimed with another Whitehall source. “We look at the Saudi strategy beforehand and we offer advice after. We have looked at what they are doing and we cannot see deliberate targeting. But mistakes happen.”

Key to the ongoing debate is, unsurprisingly, the context of UK-Saudi relations. Arms sales apart – or maybe because of them – the UK treats Saudi Arabia as a key ally in the Middle East.

“Our relationship with Saudi Arabia is extremely important,” the source said. “They are our most important ally in the Middle East. We cannot say to them: ‘We don’t trust you to investigate these things yourself.’ We wouldn’t do that to other allies like the US. It would be double standards.”

When Saudi Arabia launched airstrikes in March 2015, the expectation was that, armed with all that expensive and much-hyped precision weaponry, it would all be over quickly. It did not turn out like that. Riyadh has had to begin replenishing its stocks. Bad news for Yemeni civilians. Good news for British arms companies.